TOXIC SUBSTANCES FROM COAL COMBUSTION-FORMS OF OCCURRENCE ANALYSES. INCLUDES SEMIANNUAL REPORT FOR MARCH 31, 1998.

Detailed information on trace-element modes of occurrence in coal is essential to understanding and predicting trace-element transformations taking place during coal combustion. The USGS has developed quantitative and semi-quantitative methods for determining the mode of occurrence of trace elements in coal. This information is needed to generate predictive models for trace-element behavior, the ultimate goal of this integrated study. While the USGS has a relatively small part of the overall study, the results have direct bearing on the predictive equations being developed as the primary product of this program. The USGS procedure uses an iterative selective leaching protocol on whole coals in combination with a range of complementary studies. The sequence of leaching steps, and the intended results, are as follows: (1) exchangeable cations, and a fraction of the carbonate-hosted cations are removed by ammonium acetate; (2) cations primarily associated with carbonates and monosulfides such as galena, sphalerite and chalcopyrite are removed by hydrochloric acid; (3) silicate-associated cations are removed by hydrofluoric acid; and (4) elements associated with di-sulfides (pyrite and marcasite) are removed by nitric acid. The amount of an element leached by a given reagent is compared to concentrations of that element in the whole coal to obtain the fraction of an element having the indicated residence. Elements not leached by any of the four reagents may be present in the organic portions of the coal, or in insoluble phases such as zircon or titanium dioxides. Additionally, where mineral grains are completely encased by the organic matrix, these ''shielded'' grains may not be completely digested. Quantitative results are obtained by interpreting the leaching data together with the results obtained by complementary techniques such as electron microprobe analysis